1. Field of the Invention
The present invention relates to projection type display systems for an enlarged projection onto a screen through a projection lens by modulating a liquid crystal panel in accordance with video data. Especially, the present invention relates to a small-sized, highly portable, projection type display system using a liquid crystal panel.
2. Related Technical Art
In a small-size, highly portable, projection type display system (e.g., a projection type display system using a liquid crystal panel as an optical modulation means), the projection lens has its focal length adjusted either manually by using mechanical means or automatically by using an electric motor operated by a switch, because the distance between the projection type display system and a screen changes each time the display system location changes.
This is specifically described with reference to FIG. 8. FIG. 8 is a diagram showing the structure of a focusing mechanism for adjusting the focal point of a projection lens using an electric motor operated by a switch.
If an operation switch 504 is depressed, pulses having a width proportional to the length of time the switch is depressed, are generated by a motor drive signal generator 502. As a result, a projection lens drive motor 501 drives a projection lens 505 for a length of time corresponding to the pulse width so that the projection lens 505 is driven toward a screen 507 to change its focal length.
If an operation switch 503 is depressed, projection lens 505 is driven, in the same manner as described above, away from screen 507 to change the focal length.
Using the operations as described thus far, operation switches 503 and 504 are operated in order to move projection lens 505 and alter the distance between projection lens 505 and screen 507 to a value equal to the focal length of projection lens 505. As a result, light modulated by an optical modulation block 506 in accordance with video signals is clearly projected onto screen 507 on an enlarged scale by projection lens 505. Reference numeral 508 designates a power supply for the motor drive signal generator 502.
Since the focal adjustment is accomplished by depressing operation switches 503 and 504, according to this background art, the switches are continuously depressed while observing the imaged focused on screen 507, until the focus is adjusted to be in the vicinity of an optimum focal adjustment point. After this, the projection lens forward operation switch 504 or the backward operation switch 503 are also depressed several times while observing the on screen image until the optimum focal point is reached. This causes problems in operating the system because this requires depressing operation switches 503 and 504 several times until the optimum focal point is set.
If a user with poor visibility is adjusting the focal point, the background art finds it difficult to obtain the optimum focal point because the user has to observe the image focused on screen 507 during adjustment.